The present invention relates generally to sheeting used for signs and the like to convey information in the form of indicia. More particularly, the invention relates to such sheeting that is retroreflective.
The reader is directed to the glossary at the end of the specification for guidance on the meaning of certain terms used herein.
It is known to use retroreflective sheeting having an image thereon for image-displaying members of road signs, billboards, and the like. The retroreflective sheeting is generally either of the beaded variety or of the cube corner variety. Beaded sheeting utilizes miniature beads composed of glass or other transparent materials in connection with a reflective coating. Cube corner sheeting utilizes a microreplicated structured surface having a multitude of miniature reflective faces arranged in sets of three approximately mutually perpendicular faces, each such set of three faces referred to as a cube corner element. Except for certain pavement marker applications, such retroreflective sheeting is generally designed to have maximum retroreflective luminance for an entrance angle of about zero (0) degrees, i.e., for a light source located along an axis that is perpendicular to the sheeting. The retroreflective luminance then tends to decrease when the entrance angle exceeds a certain value.
When an image is formed on such sheeting by conventional methods, i.e., by providing a pigmented or otherwise colored layer on selected portions of the front surface of the retroreflective sheeting to form indicia, the visibility of the image deteriorates for illumination by light having a relatively high entrance angle. Thus, in order to realize good visibility of the image, the range of possible entrance angles of the light source tends to be limited. This makes it difficult to assemble a sign-displaying system so that a light source is placed near the edge of a sign surface (imaging surface) of a road sign for illuminating the sign.
Attempts have been made to enhance the retroreflective luminance of retroreflective sheeting at high entrance angles by forming an uneven front surface of the sheeting. Examples can be found in JP-A-53-46363 (Kuwata et al.), JP-A-53-46371 (Kuwata et al.), JP-A-55-65224 (Omori et al.), JP-A-57-10102 (Izutani), JP-A-57-193352 (Seki et al.), JP-Y-62-41804 (Okuno et al.), U.S. Pat. No. 4,605,461 (Ogi) and U.S. Pat. No. 4,758,469 (Lange), and PCT Publication Nos. WO97/01677 (Bacon et al.) and WO97/01678 (Bradshaw). The unevenness tends to improve retroreflectivity at high entrance angles. However, it is difficult to form an image with good visibility on the uneven front surface for forming image-displaying sheets. Also, the retroreflective luminance near zero (0) entrance angle tends to decrease.
U.S. Pat. No. 5,657,162 (Nilsen et al.) discloses a retroreflective sheeting comprising cube corner prismatic reflectors, a metallic reflective coating such as aluminum formed on only some of the prismatic surfaces, and a colored adhesive layer in contact with the rest of the prismatic surfaces. If the non-metallized surfaces were appropriately sized and configured, the adhesive layer could be made to form an image area visible from the front flat side of the retroreflective sheeting. Although the areas other than the image area could be seen at a higher luminance than the image area for light incident at a relatively low entrance angle, it is difficult to increase the visibility of the image area since it only reflects diffusely.
Retroreflective image-displaying sheets (e.g. for signs) are known, wherein a first retroreflective sheet forms the background and a second retroreflective sheet forms the image or indicia region. See, for example, U.S. Pat. No. 4,726,134 (Woltman) and U.S. Pat. No. 5,050,327 (Woltman). The two different types of sheets--one for the background, one for the image portion--have different retroreflective luminance properties as a function of entrance angle, and thus the visibility can be increased in a wider incident angle range than when only one retroreflective sheet is used. However, due to limitations of both types of sheeting at high entrance angles, the luminance cannot be increased in the relatively high entrance angle range (e.g., 70.degree. or more), and thus the visibility tends to deteriorate in such a high incident angle range.
U.S. Pat. No. 4,025,159 (McGrath) discloses a combination cube comer/exposed bead lens product. Reportedly, such combination products have improved retroreflective performance at large entrance angles at which the retroreflectivity of simple cube-comer sheeting normally falls off rapidly. Further, the smooth front surface is compatible with formation of an image layer by the conventional methods discussed above. However, such sheeting increases the luminance across the entire front surface of the sheeting, and does not improve the visibility of the image by formation of an effective difference between the retroreflective luminance of the imaging part (i.e. the indicia such as characters, designs, and areas having characters or designs) and the retroreflective luminance of the background area so as to enhance contrast of the viewed image.
If the McGrath sheeting mentioned above is used only for indicia in a construction such as Woltman '134, the contrast of the image can be increased, but achieving sufficient luminance in the high entrance angle range remains difficult.
A need exists in the retroreflective sheeting art for an image-displaying sheet and system that has good contrast between the indicia portion and the background portion, that would avoid the characteristic decrease of retroreflective luminance at high entrance angles (typically at least 70.degree.), and that would have improved visibility of the image for light incident at such high entrance angles.